1. Field of the Invention
The invention relates to a method of imaging an object placed in a static magnetic field by means of magnetic resonance (MR), which method comprises the generation of a plurality of sequences, each sequence comprising
a) excitation of nuclear spins in a portion of the object, and PA1 b) generation of a read gradient and other gradients for measurement of MR signals along a plurality of lines in k-space. PA1 means for determinating a phase correction from the navigator MR signals measured for correction of the MR signals measured, and PA1 means for correcting the MR signals measured.
The sequence furthermore comprising the generation of a first auxiliary gradient before the read gradient and a second auxiliary gradient after the read gradient, the first and the second auxiliary gradients being generated such that the nuclear spins excited in a sequence (n) are rephased in a sequence (n+i) for i.gtoreq.1, the method also comprising the reconstruction of an image of the portion of the body from the MR signals measured. The invention also relates to an apparatus for carrying out such a method.
2. Description of Related Art
Such a method is known from the article "A Fast Gradient-Recalled MRI Technique with Increased Sensitivity to Dynamic Susceptibility Effects" by C. T. W. Moonen et al., Magnetic Resonance in Medicine, Vol 26, 1992, pages 184-189. It is to be noted that in the context of the present Patent Application a k-space is to be understood as a spatial frequency domain in which an MR signal is measured along a predetermined trajectory and the measured values yield the inverse fourier transformed values of the image of the object. The trajectory in the k-space is determined by the time integral of temporary gradient fields over a time interval from the excitation of the nuclear spins to the actual instant of measurement of the MR signal. Furthermore, gradients are to be understood as temporary magnetic fields which are superposed on the steady magnetic field and cause a gradient of the static magnetic field in three respective orthogonal directions. Generally speaking, the gradient in a first direction is denoted as a read gradient, the gradient in a second direction as a phase-encoding gradient and the gradient in a third direction as a slice-encoding gradient.
The known method is a fast imaging method based on gradient recalled echoes of nuclear spins whose excitation and echo formation are not in the same sequence. Furthermore, the known method has an increased sensitivity to variations in the time constant T.sub.2.sup.* of the free induction decay of the MR signal. Therefore, the known method is used in functional MR imaging studies that are based on temporary changes in T.sub.2.sup.* which result from local changes in magnetic susceptibility, for example, local changes in the brain oxygenation state of a human or animal body.
A drawback of the known method is that image quality is susceptible to motion, such as the heart rhythm, respiration and motion of the human body itself.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.